A crystallographic description of experimentally identified formation reactions of Cu(In,Ga)Se 2

Abstract

This work describes solid-state reactions for the formation of the chalcopyrite compounds CuInSe 2, CuGaSe 2 and Cu(In,Ga)Se 2 on atomic scale. The most important chalcopyrite formation reactions which were identified by the authors by real-time in situ X-ray diffraction in preceding experiments are (A) CuSe+InSe→CuInSe 2, (B) Cu 2Se+2 InSe+Se→2 CuInSe 2 and (C) Cu 2Se+In 2Se 3→2 CuInSe 2. During the selenistaion of a metallic precursor containing gallium a separate fourth reaction occurs: (D) Cu 2Se+Ga 2Se 3→2 CuGaSe 2. The quaternary compound is finally formed by interdiffusion of CuInSe 2 with CuGaSe 2 (E). These five reactions differ in their activation energy and reaction speed. We explain these differences qualitatively by analysing the involved crystal structures for each reaction. It turns out that all reactions involved in the formation of Cu(In,Ga)Se 2 are promoted by epitaxial relations, which facilitates the formation of polycrystalline thin films at temperatures much below those necessary for single crystal growth. Recommendations for the growth of larger grains of Cu(In,Ga)Se 2 containing fewer defects are given.